A back support and a seat support should perform many important functions to provide the best benefit to a seated individual. The back and seat supports should be comfortable and encourage proper support for optimal posture and posture control for a considerable length of time. The back and seat supports should also create stability and a sense of secure confinement, thereby avoiding a feeling or tendency of falling or slipping out of the seating device. The security of confinement is particularly important to a wheelchair user. In addition, the back and seat supports should also enhance the functional capabilities of the wheelchair user by supporting independence in activities of daily living.
The seat support plays an important role in obtaining these benefits. The seat support must receive and distribute the weight of the user, while facilitating reasonable movement of the user. Because the seat support resists most of the user's weight, it should prevent and reduce the incidence of localized areas of discomfort and soreness, which in the case of wheelchair users can lead to pressure ulcers and skin shear force abrasions, created by prolonged sitting without adequate pressure relief. Pressure ulcers and shear abrasions can become a serious health problem for wheelchair users who must remain constantly in contact with the seat support.
There are a number of different support contours and configurations, as well as support theories and techniques, for configuring the contour of the seat support. One particularly advantageous configuration is described in U.S. Pat. No. 7,216,388, which is assigned to the assignee of this invention. However and regardless of the type of contour which may be employed, it is essential that the upper leg and pelvic skeletal structure and pelvic tissue of the user interact in a consistent intended way with the contour of seat support. Otherwise, the pelvic area of the user may move into an unintended position, causing distortion of the intended anatomical interaction with the contour of the seat support and diminishing or destroying the benefits desired from the seat support. This particular constraint is particularly critical for wheelchair users who often lack the capability to adjust or move their own position within the seat support and must depend entirely on the contour of the seat support and the back support to maintain the intended orientation and interaction.
The back support must function in conjunction with the seat support to maintain the optimal pelvic alignment. The back support should contact the back of the pelvic area and transfer force to restrain the pelvic area from tilting backward on the seat support. Achieving the desired optimized pelvic alignment is possible only because the back support contacts the pelvic area to hold it in the desired orientation. By itself, the seat support is substantially incapable of preventing the pelvic area of the user from tilting backward. The back support must therefore perform the essential role of maintaining a safe, comfortable, secure and desired pelvic alignment with the seat support.
In addition, the back support should play an additional important role of balancing the upper torso of the user in a comfortable and supportive manner over the pelvic area. The pelvic area is principally responsible for transferring the weight of the upper torso of the user to the seat support, so balancing the weight above the pelvic area is the most effective manner by which to transfer that weight. Without balancing the upper torso of the user above the pelvic area, the user may experience the sensation of the upper torso falling forward or the pelvic area sliding forward off of the seat support. Furthermore, not properly balancing the weight of the upper torso over the pelvic area will create a distortion to the desired alignment and interaction of the pelvic area with the seat support, and over time will diminish or destroy the desired effects of proper pelvic alignment with the seat support, leading to the health and safety concerns described above.
In many circumstances involving previous seating devices, it is impossible to position the back support to maintain proper alignment and orientation of the pelvic area and also orient the back support to balance the weight of the user's upper torso. Adjusting the back support to maintain the optimal orientation of the pelvic area did not permit balancing the weight of the upper torso over the pelvic area, causing the user to experience the sensation of falling forward or sliding forward off of the seat support. Similarly, if the back support was adjusted to balance the weight of the upper torso, the pelvic area was not maintained in the optimal alignment, resulting in an increase in discomfort and fatigue, and in the case of a wheelchair user, possible dangerous pressure ulcers and shear force abrasions, as well as reductions in the desired optimal posture, posture control, support, stability, range of motion, balance, and comfort, among other things. Consequently, it became necessary to compromise one or both of the optimal pelvic alignment or the upper torso orientation.
Attempts have been made in the past to address some of these issues. One particular device employs a back support having a cushion with a contour that projects forward at a location adapted to support the pelvic area while providing a limited range of back support orientation to assist locating the upper torso weight over the pelvic area. Another particular device uses hinges which allows a bottom portion of the back support to pivot relative to an upper portion of the back support, thereby allowing the bottom portion of the back support to interact with the pelvic area while the upper portion of the back support interacts with the upper torso of the user. Both of these prior devices provide greater flexibility in adjusting the position of the upper torso for weight balance over the pelvic area, but both of these prior devices fail to provide the most desirable benefits for interacting with the tissue and skeletal structure of the pelvic area.